The use of multi-phase brushless dc motors to power computer disk drives is well known. In such drives, a plurality of stator coil elements are uniformly arranged on a stationary substrate. A rotor assembly, having a plurality of permanent magnets, rotates about the stator coils. The coils are periodically energized in order to induce rotational movement of the rotor assembly.
Unlike conventional brush motors having mechanical brushes to commutate the stator coils, brushless or commutator-free motors used in disk drives are electronically commutated through analysis of coil waveforms, or based upon outputs from magnetic or optical sensors.
Brushless motors tend to generate unwanted noise and vibration as a result of excessive spindle power consumption. Further, while conventional dc motors have commutator bars that provide a relatively smooth torque output curve, brushless motors have relatively few phases driving the rotor. The relatively long duration of passage of the rotor over each stator coil results in significant torque ripple that, in turn, causes acoustic noise and vibration.
The prior art includes a number of teachings regarding methods for control of acoustic noise in a disk drive, under normal operating conditions. For example, U.S. Pat. No. 5,886,489 to Rowan et al., assigned to the same assignee as this application, discloses a method for reducing spindle power and acoustic noise in a brushless dc motor for a disk drive. The Rowan et al. system adjusts the commutation angle of the motor until a point of optimum power consumption is reached. Varying the commutation angle in opposite directions enables a point of minimum spindle power consumption to be determined, and enables a reduction in the spindle power and acoustic noise.
U.S. Pat. No. 5,319,289 to Austin et al. describes a circuit for providing an adaptive commutation delay in a brushless dc motor. The circuit adjusts the commutation delay as a function of the rotor's rotational speed so as to maximize motor torque and minimize power consumption. A variety of further teachings regarding commutation control of dc motors may be found in the following patents: U.S. Pat. Nos. 5,420,492 to Sood et al.; 5,457,375 to Marcinkiewicz et al.; 5,057,753 to Leuthold et al.; 5,530,326 to Galvin, et al.; and 5,739,652 to Sriram. None of the aforesaid prior art teachings consider conditions which occur when motor saturation is approached during speed control of a dc brushless motor.
Motor saturation occurs when the supply voltage to the motor driver, less the driver output voltage drop, equals the back emf generated by the motor plus the IR drop through the windings of the motor. Under such circumstances, an increased output of motor driver current is no longer possible. In such a case, even if a command is received to output a higher drive current, that command has little or no effect on the motor speed. This is due to the fact that the drive voltage available is insufficient to generate the required current and, in turn, the torque needed to increase the motor speed. Note that the condition which causes a problem for the waveform shaping circuitry is not merely one of absolute motor saturation, i.e., when the level of current being applied to the motor windings is at an absolute maximum. In fact, the problem normally occurs when the motor is "nearly" saturated, i.e., when the current level can be increased slightly, but not fast enough to reach the target, or command, value prior to the time when the motor should be commutated. In such a circumstance, for a waveform shaping motor driver that relies on reaching a target current prior to the next commutation, the motor driver loses its ability to commutate properly, as long as the waveform shaping mode is enabled. Consequently, the commutation action becomes distorted or disturbed. Eventually, the motor loses synchronization with the output frequency of the motor driver and disk drive failure occurs.
Accordingly, it is an object of this invention to provide a system and apparatus for control of a brushless dc motor used to safely operate a disk drive, when a saturation condition is approached.
It is another object of this invention to provide a method and apparatus for control of a disk drive motor which prevents loss of synchronization, when a saturation condition is approached.